archive

June 04, 2009

As the horn blares

If feats of logic and deduction thrill and amaze you like feats in a circus might, you should take an interest in astronomy.

Most celestial objects are so unimaginably far away that we can never hope to visit them. The brightest star in the sky, Sirius, also known as the Dog Star, is almost 9 light years away: light takes 9 years to reach us from there. Which means it would take you and I about 250,000 years in an average moon rocket to get to Sirius. Well, that's if we don't get lost en route. Because if we did, we might find ourselves heading for Andromeda, the nearest galaxy, 2 million light years away. And reaching Andromeda in that same moon rocket, believe me, will take a long time indeed.

So since we can't visit, all we know about our universe and the objects in it has to be deduced from observation. Arguably, no branch of science wrings quite as much knowledge out of tiny clues as astronomy does. Each one leads to inferences and deductions and more observations and other clues -- all in an enormous edifice of knowledge that, astonishingly, stands up remarkably well to the few chances astronomers get to confirm their hypotheses.

How do we, in fact, know how far away galaxies such as Andromeda are? And if you ask that, ask too: Are they moving? Where and how fast? What is the age of the universe?

Believe me again: These are not random questions at all. They are related in elegant, pleasing ways. And answering one leads astronomers to answers to others in deductive feats that would do Sherlock Holmes proud.

One clue was the Doppler shift, and you know this like you know the sun rises every day. Remember how, for example, the tone of a train's horn changes as it rushes past you, going from high to low? That's the famous shift. And here's the thing: By measuring how much the tone changes -- its Doppler shift -- we can figure out the speed of the train.

In the same way, the light from galaxies has certain characteristics. Those characteristics change, or are Doppler shifted, with the speed of the galaxy's motion. Just as with the train, we can measure the Doppler shift and calculate that speed. OK, so this is a mildly stupid way to divine the speeds of trains. With galaxies though, it makes a lot of sense. In fact it is the only way astronomers have.

In 1912, the astronomer Vesto Slipher used Andromeda's Doppler shift to estimate that it is moving towards us at 170 km per second. That's pretty damned fast, but don't be scared. It will be 4 billion years before Andromeda slams into us. (Your grandchildren, though, may want to find another planet to live on).

Actually, Andromeda is an exception. Most galaxies, their Doppler shifts tell us, are moving steadily away from us. Which says something very crucial for our understanding of the universe: it is expanding.

But let me return to that.

Astronomers are also always wondering how far away celestial objects are. Edwin Hubble estimated the distance to some galaxies. The technique he used is another fascinating combination of observation and deduction. Though I won't get into it here, it's based on two simple principles:

* Nature is uniform. The essential character of objects in our galaxy (the Milky Way) is the same as for similar objects in other galaxies (like Andromeda).

* Fainter galaxies are generally farther away.

These are reasonable assumptions, and all our observations are consistent with them. While they do mean Hubble's estimates of distance had a definite margin of error, they were good enough to make inferences from.

And so it was that Hubble discovered something most interesting about his distance estimates: The farther away a galaxy, the faster it is speeding away from us, as measured by its Doppler shift. For every 1 million light years further a galaxy is from us, Hubble found that its speed increases by about 170 km per second. Modern measurements place that figure -- it's now called the Hubble constant -- at only about 15 km per second, but the relationship between distance and speed that Hubble discovered still holds.

Given a little thought, this relationship is not surprising at all. Remember the universe is expanding. Imagine a chessboard with a person on each square, expanding evenly in all directions. You -- the black Queen, let's say -- would see the others on the board moving away from you; in fact, everyone on the board would see the others -- Ms Black Queen included -- moving away from them. What's more, people two squares from you would be moving twice as fast as your immediate neighbours, since there are two squares, both expanding, between you and them. In fact, the further away someone is from you, the faster they are moving away from you.

Which is exactly what Hubble found was happening with the objects in our universe. So a galaxy's Doppler shift, by telling us how fast it is moving, also tells us how far it is from us.

If the universe is expanding, and if there is a measure -- the Hubble constant -- of how fast it is expanding, you're probably already asking a simple question. This expansion must have started somewhere, sometime. OK, so when?

Astronomers have a name for that moment: the Big Bang, when all the galaxies in the universe exploded away from each other. When, in a mind-boggling cataclysm, the universe was born.

So when was the Big Bang?

That's easily calculated from the Hubble constant, 15 km per second per million light years. The Big Bang, it turns out, happened about 20 billion years ago.

To me, what's fascinating thing about this whole chain of reasoning is how the Doppler shift tells us so much more than just the speed of a galaxy. Yet there's still more.

If a galaxy has a very large Doppler shift, we know from Hubble that it is very far away. And if it is very far away, its light has taken a very long time to reach us. And that means such galaxies are very old indeed. The Doppler shift, you see, tells us the age of galaxies as well.

In a curious way then, when we look at distant galaxies, we are looking back through time. Think of Sirius, just 9 light years away. When you look up at it in the night sky tonight, the light you see coming from it actually left Sirius about the time we clocked over to a new millenium: when the odd term "Y2K" was a familiar and even feared one.

In that sense, most celestial objects represent a time many years ago. In fact, the Doppler shifts of the faintest, furthest galaxies we know of tell us that they are almost as old as the universe itself: 20 billion years.

Imagine that. In looking at them, we are looking at the very beginning of the universe. At the very beginning of time. At that apocalyptic convulsion, the Big Bang.

The next time the Thiruvananthapuram Rajdhani rushes past you, horn blaring, give that a thought.

4 comments:

Good stuff Dilip! Fraser Cain and Pamela Gay's fantastic AstronomyCast podcast is a great listen for astronomy beginners and enthusiasts alike.I've been trying some Astro-poetry myself on my page actually:)